The field of art to which the invention appertains is the sealing with ceramic material of electrical equipment, and more particularly sealing with ceramic material which can be subjected to high temperatures, for example in electrical heating elements.
It is necessary, with ceramic materials suitable for this purpose, to have an annealing or setting stage during which the material is heated, whilst in the position of use, to cause it to expand and bond itself to the support, and to take on the characteristics of glass.
It is desirable that the ceramic material shall not bubble unduly, and in particular shall not foam away, during the annealing stage, and Underwriters Laboratory specification in this respect is that the material must bubble a minimum one-sixteenth of an inch.
It is also desirable that the material, after annealing, shall have a relatively high impact resistance, so as to be readily capable of standing up to shocks incurred during use of the electrical equipment.
It is further desirable that the material shall have a relatively high resilience to permit it to stand up to any subsequent forming or other machining operations which may be carried out on the equipment.